Fibrous cellulosic material such as paper tissue may be composed of a single ply or multiple plies. Secure attachment of plies in a multiple ply material poses problems. One problem associated with multiple-ply tissue products is that the plies can separate while being pulled from a roll. Such separation is particularly inconvenient when a multiple-ply product such-as, for example, multiple-ply bathroom tissue is used in institutional multi-roll dispensers which are locked to prevent tampering with the tissue rolls. Separated plies of tissue may become caught in the locked dispenser causing jams or other problems.
In the past, plies have been attached by methods which rely on, for example, adhesive bonding, certain forms of crimp-bonding and/or hydrogen bonding (also called "paper" bonding). Conventional adhesives may be unsuitable for some tissue products and may add expense. Hydrogen bonding which may be created by applying water onto tissue plies can be difficult to adapt to some high-speed manufacturing processes and may produce an unsatisfactory product. Conventional mechanical crimp-bonding techniques (i.e., linear edge crimping) utilize pressure loaded, relatively narrow, hardened-metal patterned crimp wheels and smooth, hardened-metal anvil wheels to create autohesive attachment between plies at the bond points (i.e., attachment between the constituent material of the plies without application of adhesive agents).
Crimp-bonding is created when superposed plies are subjected to relatively high pressures at the bond point. Conventional crimp-bonding processes utilizing crimping wheels and anvil rolls have generally been limited to producing continuous linear bond patterns. These linear bond patterns are usually located along one or both edges of a laminate joining the plies. Conventional crimp-bonding processes are poorly suited for joining plies of material having large widths because bonding is limited to a generally continuous linear configuration intended to be near the edges of a finished sheet. Such a limitation presents a problem because high-speed multiple ply tissue manufacturing processes utilize increasingly wider rolls of material to improve efficiency. For example, some processes employ rolls of tissue having widths over 10 feet.
Simply setting up an array of conventional crimping wheels and anvil wheels across a wide sheet is unsatisfactory. Crimping wheels must be accurately spaced and the linear crimp-bond patterns must maintain their spacing during continuous commercial operation, otherwise the crimp-bonds on the finished product will be misaligned when the wide sheet is slit or cut into smaller widths (i.e., the crimp-bonds would no longer be at the edges of the finished product).
Crimp-bonding processes are quite different from conventional embossing processes. Embossing conveys tissue sheets through the nip created by a hard metal pattern roll and a resilient rubber roll. Alternatively, a set of pattern-matched, intermeshing steel embossing rolls may be used. Such combinations of rolls produce deep, durable indentations in the tissue sheet. Robust bonding between plies as produced by crimp-bonding processes are usually absent. Attachment of the embossed plies, if any, may be accomplished with an adhesive. In some situations, the folding, crinkling or creasing at the embossments may provide limited ply attachment. Conventionally embossed multiple-ply tissue products generally have greater bulk and reduced physical properties (i.e., tensile strength) than a laminate of un-embossed tissue. Moreover, rolls of such bulky embossed products need unacceptably large diameters to provide desired volumes of product for many commercial applications.
Thus, a need exists for a practical process for making a fibrous cellulosic laminate having desirable levels of ply attachment. This need also extends to a practical process for making a crimped-bonded fibrous cellulosic laminate which also has acceptable bulk properties and softness without sacrificing physical strength. Meeting this need is important since it is economically desirable to adapt high-speed manufacturing processes to take advantage of efficiencies created by processing relatively wide rolls of tissue laminates.
There is also a need for an absorbent multiple-ply tissue laminate having desirable levels of ply attachment resulting from crimp-bonding produced without the use of adhesives. This need also extends to such an absorbent multiple-ply laminate which also has acceptable bulk properties and softness. For example, there is a need for a multiple-ply bathroom tissue having desirable levels of ply attachment, good bulk properties and softness as well as desirable delivery of a volume of product when wound into a roll. Meeting these needs are important since it is economically desirable to avoid using adhesives to bond the plies of an absorbent multiple-ply tissue laminate. It is also economically and environmentally desirable to provide rolls of such tissue laminate which can be wound into a standard size roll while providing a commercially acceptable volume of product that has bulk properties, softness and appearance that is acceptable to consumers.
Moreover, there is also a need for an apparatus to produce an absorbent multiple-ply fibrous cellulose laminate having desirable levels of ply attachment resulting from crimp-bonding without the use of adhesives. There is also a need for an apparatus which provides such a laminate product which is robust, avoids destructive vibration or "chattering" and which may readily accept a variety of material widths.